1. Field of the Invention
This invention relates to communication over a multiplex data communications network. In particular, this invention relates to a system for interchanging data bytes within a node between a microcontroller (MCU) and a first peripheral at a chosen clock frequency. Then within the same node, the first peripheral generates a protocol bit and then converts both the protocol bit and each bit of the data bytes from the MCU into variable pulse width modulated (VPWM) bits forming a VPWM message. After forming the message of VPWM bits, the first peripheral sends the VPWM bits to a second peripheral that converts each VPWM bit into a communications symbol (known as a J1850 symbol) conforming to the communication requirements of the Society of Automotive Engineers (SAE) established Recommended Practice J1850 (a set of technical requirements and parameters).
The second peripheral, a transceiver, drives these symbols onto a single-wire communications bus to communicate the message to other nodes connected to the bus. The second peripheral also routes a reflection of the symbols back to the first peripheral and reconverts the symbols into VPWM bits. The first peripheral than reassembling the reflected bits into bytes used in the next interchange of bytes with the MCU and to check for the achievement of arbitration if other nodes on the bus are transmitting.
2. Description of Related Art
Normally, transmitting and receiving serial bytes between the first peripheral device and the MCU require interrupts in the transfer programming. U.S. Pat. No. 5,274,636 dated Dec. 28, 1993 of Halter et al. entitled "Automatic Multiplex Data Link System, Symbol Encoder Decoder Therefor" (or SED patent) teaches such a system. Such interruptions imposes a heavy burden on the MCU to perform the process of transmitting and receiving serially arranged byte messages. During transmission, programming of the MCU must allow for, illustratively, 34 microseconds between bytes for the introduction of an interrupt. This imposes time restraints on the MCU when preparing each byte for transmission. Similar time restraints occur for receiving bytes of data. We searched for ways to reduce the imposed time restraints on the MCU while improving the processing performance. That search ended in the present invention which accomplishes reducing the time restraints required to process the data prior to converting the data into J1850 symbols and for determining whether a node has gained control of the bus by winning an arbitration contest with competing nodes during a message frame.